The present invention relates to a novel agent and resultant compositions that can be used to retain product quality and improve shelf life of oxygen sensitive materials, and to intermediate shaped structures, e.g., films, coatings, 3-dimensional solids, fibers, webs and the like which contain said composition, as well as to shaped products into or onto which said composition or structure are incorporated or applied to, be part of or attached to the container structure.
The present agent is a modified anionic hydrotalcite-like particulate. It may be formed into a composition composed of said agent in a carrier which permits the agent to combine with oxygen when in the presence of moisture. Specifically, the composition utilizes modified anionic hydrotalcite-like particulates which have certain anionic groups, as fully described herein below. The particulate containing oxygen scavenging composition of the present invention has unexpectedly been found to provide effective absorption of oxygen from the interior of a container without adversely affecting the color, taste or smell of the packaged products contained therein which is normally associated with conventional agents and/or oxidation by-products thereof.
The subject oxygen scavenging composition has the ability to effectively chemically combine with oxygen in contact therewith, such as from the interior of a container, without undue migration of the oxygen scavenging agent or its oxidation by-product(s) out of the composition's matrix. The inhibition of migration is of particular advantage in that it significantly reduces or eliminates adverse effects on the color, taste, or smell of articles in contact with the matrix composition as well as provides a means of using high levels of scavenging agent while meeting government regulations directed to amounts of extraneous material permitted in food products.
In order to enhance preservation, it is standard practice to package food and other materials within laminated packaging material that generally includes a barrier layer, that is, a layer having a low permeability to oxygen. The sheet material can be thin, in which event it is wrapped around the material being packaged, or it can be sufficiently thick that it forms a shaped container body that is provided with a lid or other separate closure. The polymeric sheet material may constitute some or all of the interior exposed surface area of the container or its closure means.
It is known to include an oxygen scavenger agent in sheet material. The oxygen scavenger agent reacts with oxygen that is trapped in the package or that permeates into the package. This is described in, for instance, U.S. Pat. Nos. 4,536,409 and 4,702,966 and the prior art discussed in these references. U.S. Pat. No. 4,536,409, for example, describes cylindrical containers formed from such sheet material and provided with metal lids.
When the container is formed of a glass or metal body and is provided with a hermetically sealed metal closure, the permeation of oxygen through the body and the closure is theoretically impossible because of the impermeability of the materials forming the body and closure. As a practical matter, metal cans can reliably prevent oxygen ingress. However, some oxygen ingress may occur by diffusion through the gasket or the like positioned between a container body and its lid. It has long been recognized that when conventional containers of these types are used for the storage of oxygen sensitive materials, the shelf life of the stored materials is very limited. The quality of the packaged material tends to deteriorate over time, in part because dissolved oxygen typically is present in the pack from the time it is filled; and in part due to oxygen ingress which occurs during storage.
When the container is in the form of a can, the can end or other closure in many instances includes push components or pull components which are intended to be, respectively, pushed or pulled in order to allow removal of the fluid or other material in the container without removing the entire closure from the container. These push or pull components are often defined by discontinuities or lines of weakness in the panel of the closure. Problems that can arise at these lines of weakness or discontinuities include the risk of permeation of oxygen into the container and the risk of corrosion of the metal where the normal protective lacquer coating is ruptured at the lines of weakness or at the discontinuities.
It would be very desirable to be able to significantly improve the shelf life while continuing to use conventional materials for the formation of the container body, the container closure and, where applicable, the gasket between the body and closure.
Various types of oxygen scavengers have been proposed for this purpose. For example, it is well known to package iron powder in a sachet for use with dry foods. See Mitsubishi Gas Chemical Company, Inc.'s literature titled "Ageless.RTM.--A New Age in Food Preservation" (date unknown). However, these materials require the addition of water soluble salts to enhance the oxygen scavenging rate and, in the presence of moisture, the salts and iron tend to migrate into liquids, producing off-flavors. Similarly, U.S. Pat. No. 4,536,409 issued to Farrell et al. recommends potassium sulphite as a scavenger, with similar results. U.S. Pat. No. 5,211,875 issued to Speer et al. discloses the use of unsaturated hydrocarbons for use as oxygen scavengers in packaging films.
It is known in the art that ascorbate compounds (ascorbic acid, its alkali metal salts, optical isomers, and derivatives thereof) as well as sulfites, bisulfites, phenolics, etc. can be oxidized by molecular oxygen, and can thus serve as an oxygen scavenging material, for example, as a component of a closure compound. For example, U.S. Pat. No. 5,075,362, issued to Hofeldt et al., discloses the use of ascorbate compounds in container closures as oxygen scavengers.
U.S. Pat. No. 5,284,871 issued to Graf relates to the use of an oxygen scavenging composition made of a solution of a reducing agent and dissolved species of copper which are blended into foods, cosmetics and pharmaceuticals. Copper ascorbate is used in the examples. The reference indicates that relatively high level of Cu.sup.2+ (.about.5 ppm) are required in the food for scavenging to be effective but indicates that small amounts of the Cu.sup.2+ can combine with oxygen in food to cause food spoilage. In order to avoid spoilage, one is required to reduce the amount of headspace O.sub.2 or partially flush the container with an inert gas (Col. 5, lines 32-39). A paper by E. Graf, "Copper (II) Ascorbate: A Novel Food Preservation System", Journal of Agricultural Food Chemistry, Vol. 42, pages 1616-1619 (1994) identifies copper gluconate as a preferred raw material.
It is also well known in the scientific literature (See "Polymer Compositions Containing Oxygen Scavenging Compounds", Teumac, F. N.; et al. WO 91/17044, published Nov. 4, 1991, filed on May 1, 1991) that the oxidation rate of ascorbate compounds can be increased significantly by the use of catalysts. Typical oxidation catalysts for ascorbic acid and its derivatives are water soluble transition metal salts. When such catalysts are combined with an ascorbate compound in a polymeric matrix, e.g., a PVC closure formulation, they are effective in catalyzing the oxidation of the ascorbate compound, and increase the oxygen scavenging rate of the ascorbate.
In each of the above references, the active component of the oxygen scavenging systems utilized agents which readily transfer into the food or other packaged product or materials or which produce oxidation by-products which are known to adversely affect a wide range of packaged material.
Hydrotalcite is a naturally occurring mineral commonly classified as a clay. Generally speaking, clays break down into broad groups of being cationic materials which are commonly found in nature or anionic materials, which is rarely found in nature. These materials are used in a wide range of applications, such as industrial absorbents, catalysts, fillers, decolorizing agents and the like. Naturally occurring hydrotalcite are hydroxide-carbonate minerals of the formula EQU Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3.4H.sub.2 O
It is well known that the hydrotalcite mineral is strongly bonded to the carbonate. The carbonate may be driven off by thermal calcination.
Recently, hydrotalcites with anions other than carbonate have been synthesized. They are generally layered double hydroxides (LDH) which include anionic hydrotalcite-like compounds (HTLC). They have been described in U.S. Pat. Nos. 5,399,329 and 5,507,980 as well as by W. T. Reiche in Chem Tech (1986) 58-63, the teachings of which are incorporated herein in their entirety by reference. These new materials have anions in its crystal structure which are easily exchanged. Although the known HTLC may have variations in the cationic and anionic groups, these materials have not been suggested as oxygen scavenger materials nor, in general, found useful in food packaging applications.
It is highly desired to provide an effective oxygen scavenging material and system which has good oxygen absorption capabilities and capacity and which does not adversely effects the color, taste or smell of the packaged material.
It is further desired to provide a material and system which is capable of inhibiting the release of oxidation by-product(s) which may adversely effect the color, taste or smell of the packaged material.
It is further desired to provide an effective oxygen scavenging system which has the active scavenger agent contained within a carrier and the agent still provides effective scavenging capacity.
It is further desired to provide an effective oxygen scavenging system which is thermally stable and, thereby, capable of allowing the packaging system to be formed by conventional techniques which include elevated temperature processing steps.